This invention relates generally to the field of rigid magnetic disc drives, and more particularly, but not by way of limitation, to a test head configuration which is capable of both disc media magnetic certification and thermal asperity detection.
Disc drives of the type known as "Winchester" disc drives or hard disc drives are well known in the industry. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent to the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and recording media have been made.
For instance, the first rigid disc drives used in personal computers had a data capacity of only 10 megabytes, and were in the format commonly referred to in the industry as the "full height, 51/4" format. Disc drives of the current generation typically have a data capacity of over a gigabyte (and frequently several gigabytes) in a 31/2" package which is only one fourth the size of the full height, 51/4" format or less. Even smaller standard physical disc drive package formats, such as 21/2" and 1.8", have been established. In order for these smaller envelope standards to gain market acceptance, even greater recording densities must be achieved.
The recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricatd using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. This technology path was necessitated by the need to continuously reduce the size of the gap in the head used to record and recover data, since such a gap size reduction was needed to reduce the size of the individual bit domain and allow greater recording density.
Since the reduction in gap size also meant that the head had to be closer to the recording medium, the quest for increased data density also lead to a parallel evolution in the technology of the recording medium. The earliest Winchester disc drives included discs coated with "particulate" recording layers. That is, small particles of ferrous oxide were suspended in a non-magnetic adhesive and applied to the disc substrate. With such discs, the size of the magnetic domain required to record a flux transition was clearly limited by the average size of the oxide particles and how closely these oxide particles were spaced within the adhesive matrix. The smoothness and flatness of the disc surface was also similarly limited. However, since the size of contemporary head gaps allowed data recording and retrieval with a head flying height of twelve microinches (0.000012 inches) or greater, the surface characteristics of the discs were adequate for the times.
Disc drives of the current generation incorporate heads that fly at nominal heights of only about 2.0.mu.", and products currently under development will reduce this flying height to 1.5.mu." or less. Obviously, with nominal flying heights in this range, the surface characteristics of the disc medium must be much more closely controlled than was the case only a short time ago.
Industry demands for increasing data storage capacity are being met by increases in the areal density with which data are stored on the disc surfaces. The areal density of a disc drive product is defined as the mathematical product of the linear density (or number of bits recorded along the length of the data track), typically defined as "bits per inch", and the track density, measured radially across the disc and defined in "tracks per inch".
In order to increase the areal density at the current industry rate of approximately 60% per year, the track density is constantly being increased, and in order to accomplish this, the width of the operational read/write heads has steadily decreased, with current disc drive products incorporating heads having a width of 2.0 microns, 1.5 microns or less. It will be apparent to one of skill in the art that the decrease in track width leads, in turn, to a decrease in the size of an allowable disc media defect.
Each disc is statistically tested at the component level before being assembled into a disc drive. Magnetic defects are tested for in a process called "certification testing". During the disc certification process, a selected test signal is written to the disc and then read back. If the amplitude of the recovered signal falls below a predetermined level, a defect is recorded. Allowable defects are typically on the order of 33% of the intended track width. As the track widths decrease, so, too, does the size of allowable defects. With a 2.0 micron product track width, the allowable defect size is 0.7 microns. As the product track width decreases to 1.5 microns, the allowable defect size will be less than 0.5 microns.
The heads used to perform disc certification testing are referred to as "certification heads". During current certification testing, the certification heads cover nominally 35% of the disc surface. As the allowable defect size decreases, the width of the certification heads must also decrease accordingly, and the test time needed to maintain 35% coverage of the disc surface increases proportionally. As the time to certify each disc increases, the throughput of each individual test unit does down, in turn increasing the cost of certifying each disc. The currently acceptable certification time for each disc is one to two minutes.
With the incorporation of MR heads in disc drives and the decrease in head flying heights noted above, a new type of media defect called a thermal asperity, or TA, has become of concern to the industry. Such defects are referred to as "thermal" asperities because they cause non-data-related temperature variations in the MR element. These temperature variations result in resistance changes in the MR element, which in turn lead to read errors in the disc drive. Thermal asperities can be experienced in several modes, which will be discussed below.
The first mode in which TAs are exhibited can be referred to as "contact TAs". Contact TAs occur when actual physical contact occurs between the MR element of the MR head and a "high" spot on the disc surface. Such physical contact causes rapid frictionally-induced heating of the MR element, with an attendant large rapid change in the resistance of the MR element. A simplified representation of the component relationship that causes a contact TA, along with the resultant effect on the read data channel, are shown in FIGS. 1 and 2, respectively.
FIG. 1 shows a head slider 100 which includes a MR read element 102. This MR read element is sometimes referred to as a "MR stripe". The nominal surface of a disc is shown at 104, and the space 106 between the lower surface of the slider 100 and the nominal disc surface 104 represents the flying height of the slider 100. The relative sizes shown for the slider 100, MR stripe 102 and flying height 106 are not to scale and are for purposes of discussion only.
In the figure, the disc is moving relative to the slider 100 in the direction shown by arrow 108.
As the disc rotates beneath the slider 100, a high spot 110 on the disc surface passes under the MR element 102. The vertical height of the high spot 110 is large enough that contact occurs between the disc and the MR element 102. This contact causes frictionally-induced heating of the MR element 102. As is well known in the art, such heating of the MR element 102 results in a proportional increase in the resistance of the MR element 102 local to the point of contact. The effect of this frictionally-induced heating and resistance change is illustrated in FIG. 2.
As is known in the industry, a constant bias current is applied across the MR element of a MR head. In normal operation, any change in the magnetic flux on the disc surface which passes below the MR element causes a corresponding change in the resistance of the MR element. The change in resistance, in turn, causes a change in the voltage across the MR element, and this change in voltage is sensed by the data read channel.
FIG. 2 shows the nominal voltage across the MR element as a result of the bias current at 112. As can be seen in the figure, when the high spot 110 on the disc surface contacts the MR element 102, a large voltage spike 114, sometimes referred to as a "super pulse", occurs in the read data channel. The relationship between the change in temperature of the MR element and the change in voltage sensed by the read data channel can be expressed by the following relationship: EQU .delta.V.sub.TA =i.delta.R.sub.TA =iR.multidot..beta..multidot..delta.T.sub.TA
wherein .delta.V.sub.TA =the change in sensed voltage due to the TA,
i=the constant bias current PA1 .delta.R.sub.TA =the change in resistance of the MR element due to the TA, PA1 R=the nominal resistance of the MR element, PA1 .beta.=the thermo-resistance coefficient of the MR element material, and PA1 .delta.T.sub.TA =the temperature change due to the TA.
The voltage spike 114 has a very rapid rise time (on the order of 20-100 nanoseconds), and returns to its normal level over a relatively long time interval (on the order of a microsecond). This rapid rise and gradual decay of the voltage spike is indicative of the rapid rise in temperature induced by friction when the high spot 110 contacts the MR element 102, and the gradual dissipation of the frictionally-generated heat from the MR element to surrounding elements of the disc drive, as will be discussed in more detail below.
The effect of the thermally-induced voltage spike 114 on the electronics of the read data channel can be best appreciated when a comparison is made between the amplitude of the voltage spike 114 due to the contact TA and the amplitude of normal voltage variations due to magnetically-induced resistance changes caused by data recorded on the disc.
FIG. 2 shows a sine wave 116 which represents the voltage variation sensed in the read data channel as a result of a magnetically recorded constant frequency data signal. As can be seen, this normal data read signal 116 is referenced to and centered on the normal read channel voltage reference 112, and has a nominal voltage range represented by arrow 118. The read data channel logic would, therefore, be optimized to respond to and distinguish voltages within the nominal magnetic-data-induced voltage range 118.
As can be seen in the figure, the thermally-induced voltage spike 114 is significantly greater in amplitude than the nominal data voltage range 118, and the data signal 116 riding the voltage spike 114 far exceeds the expected range of voltage variation. Such a large voltage spike can be expected to saturate the read data channel logic, and, since the voltage spike endures for several cycles of recorded data, be further expected to result in several bits of "lost" data.
While the high spot 110 shown in FIG. 1 is illustrated as an integral part of the disc surface, it will be appreciated by those of skill in the art that similar contact and data recovery losses can result if the high spot 110 were to be instead a particulate contaminant of comparable size which adhered to the disc surface and passed under and contacted the MR element. It is, therefore, common in the industry to refer to contact TAs that result from integral high spots in the disc surface, such as the high spot 110 of FIG. 1, as "native" TAs, while contact TAs that result from particulate contamination after manufacture are referred to as "grown" TAs, resulting from post-manufacture particulate contamination of the disc surface.
Another mode in which thermal asperities are exhibited will be referred to as "non-contact" TAs, and will be discussed below. However, before such non-contact TAs are discussed, it is necessary to further discuss the normal conditions present in a disc drive incorporating MR heads.
FIG. 3 represents the normal relationship between various elements of the disc drive system, and shows a portion of a slider 120 incorporating a MR element 122. Once again, the relative size of the various elements of the drawing are not to scale, and have been selected for illustrative purposes only. As previously mentioned, during normal operation, a constant bias current is applied across the MR element 122. The application of this bias current results in heating of the MR element. Typical MR heads also include thermally conductive shield elements 124, 126, which may also be functional elements of the inductive write element of the head.
When the slider 120 is in its intended relationship with a disc surface 128, an air gap 130 exists between the slider 120 and the disc surface 128.
The heat generated in the MR element 122 by the application of the bias current dissipates to the shield elements 124, 126, and, to a lesser extent, across the air gap 130 to the disc as shown by arrows 132 and 134 respectively. In actuality, approximately 98% to 99% of the heat generated by the bias current in the MR element 122 is dissipated through the shield elements 124, 126, while approximately 1% to 2% of the heat is conducted across the air gap 130 and into the disc. As will be apparent to one of skill in the art, since a constant bias current is applied to the MR element 122, a state of thermal equilibrium will quickly be thus attained, allowing effective recovery of previously recorded data as a result of magnetically-induced resistance changes in the MR element 122.
Non-contact TAs occur as a result of changes in the just described thermal equilibrium, and can be exhibited in either of two modes. These two non-contact TA types will be referred to as "positive non-contact TAs" and "negative non-contact TAs" and discussed in turn below.
Turning now to FIG. 4, shown is a slider 140 incorporating a MR element 142. The slider 140 is shown flying above a disc surface 144. The nominal air gap between the slider 140 and the disc surface 144 is designated 146. When the slider 140, disc surface 144 and air gap 146 are in their nominal relationship, the thermal equilibrium described above in relationship to FIG. 3 exists. As can be seen in FIG. 4, however, a low spot 148 in the disc surface 144 is passing under the MR element 142 as the disc moves relative to the slider 140 in the direction shown by arrow 150.
As the low spot 148 in the disc surface 144 passes under the MR element 142, the distance between the MR element 142 and the disc surface increases. This increase in spacing between the MR element and the disc reduces the effectiveness of the heat dissipation between the MR element 142 and the disc which was designated as the thermal dissipation path 134 in FIG. 3. Since less heat is able to dissipate from the MR element 142 to the disc, the overall temperature of the MR element 142 rises, causing an increase in its resistance. This increase in MR element temperature will continue until either a new thermal equilibrium level is reached, or until the low spot 148 completely passes the MR element 142, at which time the overall temperature of the MR element will return to its original equilibrium level.
The effect of such a low spot 148 passing under the MR element 142 is shown in FIG. 5. In FIG. 5, the nominal non-active voltage level sensed by the read data channel as a result of thermal equilibrium is shown at 152. As seen at 154, however, as the low spot 148 of FIG. 4 begins to pass below the MR element, the voltage level begins to rise. This is a result of the increase in resistance in the MR element brought about by the increase in temperature of the MR element due to a decrease in the amount of heat dissipated to the disc from the MR element. Curve 156 shows the voltage level change as the low spot 148 of FIG. 4 passes under the MR element 142, and illustrates the return to the nominal voltage level 152 once the low spot passes beyond the MR element 142. A person of skill in the art will appreciate that, if a voltage variation representative of magnetically-induced data recovery--such as that shown at 116 in FIG. 2--were referenced to the voltage curve 156 caused by the low spot 148, the resultant signal would once again exceed the operational range of the read data channel logic, resulting in saturation of the data recovery logic and loss of any data during the period of the voltage spike 156. It is because the low spot 148 in the disc surface causes a positive voltage spike in the read data channel without direct contact between the disc and the MR element that this type of TA is referred to as a "positive non-contact TA".
FIGS. 6 and 7 illustrate the cause and result of a negative non-contact TA. In FIG. 6, a slider 170 incorporating a MR element 172 is shown flying above a disc surface 174 at a nominal flying height represented by the air gap 176 between the slider 170 and the disc surface 174. A high spot 178 in the disc surface passes under the MR element 172 as the disc rotates past the slider 170 in the direction of arrow 180. The high spot 178 has a vertical height relative to the nominal disc surface 174 which is less than the vertical dimension of the air gap 176 between the slider 170 and the disc surface 174, so that no direct contact between the MR element 172 and the high spot 178 occurs.
As the high spot 178 passes beneath the MR element 172, however, the air gap between the MR element 172 and the disc 174 is reduced, bringing the MR element 172 and the disc 174 into closer proximity. This increase in proximity allows a greater than normal amount of heat to be dissipated from the MR element to the disc, resulting in sudden increased cooling of the MR element 172. As the temperature of the MR element 172 falls, its resistance also decreases by a proportional amount. The effect of this sudden decrease in the resistance of the MR element is illustrated in FIG. 7.
In FIG. 7, numeric reference 182 represents the nominal voltage level sensed by the read data channel when the thermal equilibrium previously discussed exists and no previously recorded magnetic data are influencing the MR element. As the high spot 178 of FIG. 6 passes under the MR element 172, the increased heat dissipation and attendant reduction in resistance of the MR element 172 causes a sharp reduction in the voltage sensed in the read data channel until the high spot 178 completely passes the MR element 172 and thermal equilibrium is again attained. This causes the negatively-going voltage spike 184 of FIG. 7.
If, once again, it is envisioned that a varying voltage level, such as that designated 116 in FIG. 2, representative of recovered magnetic data is referenced to the signal 182 of FIG. 7, it is apparent that any such signal occurring during the negative pulse 184 would be expected to fall below the threshold level necessary to allow reliable data recovery, and thus cause data loss during the time interval that the high spot is passing below the MR element. It is from the negative voltage spike induced by this non-contacting variation in the disc surface that the designation "negative non-contact TA" derives.
It is a common practice in the industry to test for defects in the disc surface using precision glide test units. Such testing is performed on the discs at the component level before the discs are assembled into a disc drive, and typically involves flying a special test head at a height above the disc surface which is approximately half of the nominal flying height intended for the finished disc drive. Glide test units typically utilize linear actuators to move the test heads radially across the surface of the disc under test in order to eliminate the effects of the skew angle changes inherent in the rotary actuators commonly used in current disc drive products.
One commonly utilized test head used for glide testing of discs includes a hydrodynamic slider unit which mounts a piezoelectric crystal, hereinafter referred to as a piezo element. When a defect on the surface of the disc being tested rotates under the slider, the slider and piezo element are distorted by contact between the head and the defect on the disc, and a small voltage is generated by the piezo element as a result. Correlation of these induced voltage spikes to the actuator position and the rotational position of the disc allows a mapping of the defects on the disc surface.
Glide test heads which incorporate a piezo element cannot detect all non-contact thermal asperities, however, since some thermal asperities will not be of sufficient size to cause excitation of the test head body or the piezo element. Therefore, other means must be found to detect the presence of all thermal asperities in disc media.
One method currently used to test for TAs involves the use of a normal read/write head incorporating a MR element flown at approximately the flying height intended for the disc drive in which the disc will be incorporated. When the MR element passes one of these TAs, temperature changes in the MR element, caused as described above, induce corresponding changes in the resistance of the MR element. The change in resistance induced by thermal asperities is detectable using electronic circuitry similar to currently employed data read channels for MR heads, and, therefore, these defects can also be mapped, using actuator and spindle position correlation as will be described below.
The main drawback to the use of standard production MR heads for media defect testing is the fact that the MR elements in such heads are dimensioned to sense data recorded at current data densities. This, in turn, means that the test unit must move the test head across the disc in such small steps that the testing time for a single disc surface is unacceptably long. There is also a low but significant risk that the MR element will be damaged during such testing, thus increasing labor and parts cost.
Co-pending U.S. patent application Ser. No. 08/855,142, filed May 13, 1997 and assigned to the assignee of the present application describes a wide thermo-resistive (TR) sensor useful for detecting the presence of thermal asperities. The material of this wide TR sensor element is specially selected to optimize sensitivity to thermal asperities, and is 40 to 50 times as wide as a normal MR element in a MR read/write head. When incorporated in a glide test head, the wide TR element thus allows rapid testing of an entire disc surface for the presence of thermal asperities. Such heads are, however, inherently incapable of performing magnetic disc certification, since they are optimized for thermo-resistive sensing and thus have little or no sensitivity to magnetic changes on the disc surface. Furthermore, the extreme width of such heads precludes their use for detecting data recorded at current track densities.
A need clearly exists, therefore, for a test head which is capable of both magnetic certification and thermal asperity detection for magnetic disc recording media, and for a test head which improves the testing time for each disc.